According to the Coriolis principle, when in a system a rotating mass movement and a straight line mass movement extending, at least partially, perpendicularly to the rotational axis, superimpose, there then acts on the moved mass an additional force, which is referred to as the Coriolis force. This effect is utilized in known manner in Coriolis, flow measuring devices, for example, for ascertaining the mass flow of a fluid flowing in a pipeline. Coriolis, flow measuring devices have, as a rule, one or more measuring tubes, wherein these can, depending on type of device, be embodied in various configurations. The system of the at least one measuring tube forms an oscillatory system, which, depending on measuring tube configuration, has corresponding natural oscillation modes, such as, for example, bending oscillations (fundamental mode as well as modes of higher order), torsional oscillations (fundamental mode as well as modes of higher order), etc.
A Coriolis flow measuring device is applied for use in a pipeline, through which a medium flows, in such a manner that the fluid flows through the least one measuring tube. For determining a mass flow of the fluid, the at least one measuring tube is excited to execute oscillations by means of at least one exciter. The at least one exciter can, for example, be formed by an electromechanical exciter, especially an electrodynamic exciter, which exerts on the measuring tube a force corresponding to an applied voltage. As a rule, the oscillatory system is excited at a resonance frequency of the same (for example, the fundamental mode of the bending oscillation). If a fluid is not flowing through the at least one measuring tube, then the entire measuring tube oscillates in phase. If a fluid is flowing through the at least one measuring tube, then a Coriolis force acts on the moved mass (the fluid). This leads to the fact that, due to the Coriolis force, the measuring tube is additionally deformed, and a phase shift occurs along the direction of elongation of the measuring tube. The phase shift along a measuring tube can be detected by corresponding sensors, which can be formed by electromechanical, especially electrodynamic, sensors arranged spaced apart from one another, along the direction of elongation of the measuring tube. The phase shift, which can be registered via the sensors, is proportional to the mass flow through the measuring tube.
Additionally, or alternatively, to mass flow, the density of the flowing fluid can also be ascertained by Coriolis flow measuring devices. In such case, the principle utilized is that the resonance frequency (for example, the fundamental mode of the bending oscillation) depends on the oscillating mass, and with it, the density of the flowing fluid. By readjusting the excitation frequency in such a manner that the oscillatory system is excited in its resonance frequency, the resonance frequency, and from it, again, the density of the flowing fluid can be ascertained. Additionally, or alternatively, still other physical measured variables of the flowing fluid, such as, for example, viscosity, can be ascertained by Coriolis flow measuring devices.
Frequently, Coriolis flow measuring devices are used, which have two measuring tubes, inserted in parallel into the flow path, such that the fluid flowing in the pipeline is divided into the two tubes. As a rule, in use, the two measuring tubes are excited with opposite phase to one another. In this way, a decoupling of the oscillatory system, which has the two measuring tubes, from external vibratory influences is achieved. Additionally, the Coriolis flow measuring device can also have more than two measuring tubes, which, for example, are inserted in parallel into the particular flow path. In industrial applications, especially where high viscosity or inhomogeneous fluids are used, it can occur, that in the case of such Coriolis flow measuring devices, (at least) one of the measuring tubes becomes blocked. Such blockage is difficult to detect, since flow is still enabled through the at least one measuring tube remaining open. Especially with regard to hygienic requirements, it is, however, desirable to detect blockage of a measuring tube in Coriolis flow measuring devices as reliably and as early as possible.
In U.S. Pat. No. 7,421,350 B2, a flow measuring device is described, through which a material remaining in a measuring tube arrangement is detectable. For this, generally after emptying the flow measuring device, the measuring tube arrangement is excited to execute oscillations, and the response of the oscillations is registered. If the response of the oscillations exceeds a limit value, then it is established that residual material is still contained in the measuring tube arrangement. In accordance with a variant described in U.S. Pat. No. 7,421,350 B2, a standard resonance frequency for the flow measuring device is determined, which corresponds to a completely emptied state of the measuring tube arrangement. If, after emptying the measuring tube arrangement, the determined resonance frequency deviates from the standard resonance frequency by more than a predetermined value, then it is established that residual material is in the measuring tube arrangement. The steps explained above for determining whether the measuring tube arrangement is completely emptied, are, as a rule, first executed after emptying the flow measuring device. Accordingly, a prompt detection of blockage is not possible. Especially, in a condition in which fluid is flowing through the measuring tube arrangement, a shifting of the resonance frequency is not a reliable indicator for the occurrence of blockage. For example, such a shifting of the resonance frequency can also be brought about by a change in density of the fluid.